Polynomial texture mapping (PTM) also known as reflectance transformation imaging (RTI) is a photographic technique that is used for capturing an object's surface shape and color and enabling computerized selective viewing of the object using highly directional lighting sources positioned at specific angles relative to the object. PTM/RTI also permits the mathematical enhancement of the subject's surface shape and color attributes, to reveal surface information that is not readily observable under direct examination of the physical object with a naked eye or even with viewing aids such as optical filters, relief techniques, and the like. For purposes of this disclosure RTI and PTM are referred to interchangeably.
PTM images may be created from information derived from multiple digital photographs of an object shot from a stationary camera position. In each photograph, light is projected from a different known, or knowable, direction. This process produces a series of images of the same subject with varying highlights and shadows. Lighting information from the images is mathematically synthesized to generate a mathematical model of the surface, enabling a user to re-light the PTM image interactively and examine its surface on a screen.
As mentioned above, in order to obtain the data set for interactive PTM images, a series of photographs of the object may be captured when a light is located at a different location. In order for the images to be useful for PTM imaging, it is necessary to have knowledge of the locations of the light, and in some cases improper light locations may have a negative impact on the processing of the images. Furthermore, in some cases efficiency may be enhanced by having reliably placed lighting for a series of PTM images.